1. Field of the Invention
The present invention relates to a numerical control apparatus for a machine tool having a spindle head which applies different levels of a preload to bearings rotatably supporting a main spindle in accordance with the rotation speed of the main spindle.
2. Description of the Prior Art
Machine tools typically have a main spindle which supports a tool for machining a workpiece, and the main spindle is rotatably supported by bearings (angular contact ball bearings). In such a machine tool, a preload is generally applied to the bearings when the main spindle (tool) is rotated for the machining of the workpiece. The preload is switchably set at different preload levels according to the rotation speed of the main spindle. More specifically, a greater preload (a higher level preload) is applied to the bearings to increase the rigidity and rotation accuracy of the main spindle when the main spindle is rotated at a lower rotation speed. When the main spindle is rotated at a higher rotation speed, on the other hand, a smaller preload (a lower level preload) is applied to the bearings for prevention of seizure of the bearings which may otherwise occur due to frictional heat.
Techniques for the switchable setting of the preload level are disclosed, for example, in Japanese Unexamined Patent Publications No. 62-124805 (1987) and No. 2-279203 (1990). With these techniques, the rotation speed of the main spindle is detected by a sensor or the like, and a hydraulic mechanism is driven on the basis of the detection of the sensor, whereby the level of the preload to be applied to the bearings is switchably set according to the rotation speed. However, the main spindle is liable to be axially displaced when the preload level is changed. Therefore, the relative position of the main spindle (tool) with respect to the workpiece is changed by the change in the preload level. This disadvantageously reduces the machining accuracy.
One approach to this problem is to correct the displacement of the main spindle caused due to the change in the preload level as proposed in Japanese Unexamined Patent Publication No. 2000-84702. A spindle unit disclosed in Japanese Unexamined Patent Publication No. 2000-84702 includes non-contact displacement detection means for detecting an axial displacement of the main spindle, and movement correction means for correcting axial movement of the main spindle on the basis of the axial displacement detected by the non-contact displacement detection means when the level of the preload to be applied to the bearings is changed.
However, the spindle unit cannot correct a displacement of the main spindle occurring in accordance with the rotation speed of the main spindle, though being capable of correcting the displacement of the main spindle caused due to the change in the preload level. When the main spindle is rotated for the machining of the workpiece, the main spindle is radially expanded and axially contracted due to a centrifugal force exerted thereon, so that the position of a distal end of the main spindle is changed. Therefore, the displacement of the main spindle varies depending on the rotation speed of the main spindle. That is, the displacement becomes more remarkable, as the rotation speed increases. Unless the displacement of the main spindle occurring in accordance with the rotation speed of the main spindle is corrected, the relative position of the main spindle (tool) with respect to the workpiece is changed depending on the rotation speed. This results in reduction in machining accuracy.
It is therefore an object of the present invention to provide a numerical control apparatus for a machine tool, which is capable of correcting an axial displacement of a main spindle caused due to a change in preload level as well as an axial displacement of the main spindle occurring in accordance with the rotation speed of the main spindle.
A numerical control apparatus according to the present invention is used with a machine tool having a spindle head which applies different levels of a preload to a bearing rotatably supporting a main spindle in accordance with the rotation speed of the main spindle, and adapted to numerically control the movement of the spindle head. The numerical control apparatus comprises: a numerical control section for numerically controlling the movement of the spindle head; a spindle control section for controlling the rotation of the main spindle on the basis of a rotation command from the numerical control section; a preload change control section for changing the level of the preload to be applied to the bearing in accordance with the rotation speed of the main spindle on the basis of the rotation command from the numerical control section; and a correcting section for outputting correction data for correction of an axial displacement of the main spindle caused due to a change in the preload level and an axial displacement of the main spindle occurring in accordance with the rotation speed of the main spindle to the numerical control section on the basis of the rotation command from the numerical control section, wherein the numerical control section corrects the axial displacement of the main spindle occurring in accordance with the rotation speed of the main spindle and the axial displacement of the main spindle caused due to the change in the preload level on the basis of the correction data.
In the numerical control apparatus, axial displacements of the main spindle occurring at different preload levels and at different rotation speeds are preliminarily measured, and correction data values for the different preload levels and the different rotation speeds are stored in the correcting section. Then, the correcting section retrieves a correction data value on the basis of the rotation command from the numerical control section, and outputs the correction data value as the correction data to the numerical control section. The numerical control section corrects the axial displacement of the main spindle caused due to the change in the preload level and the displacement of the main spindle occurring in accordance with the rotation speed on the basis of the correction data value. The preload change control section judges whether the level of the preload to be applied to the bearing is to be changed on the basis of the rotation command from the numerical control section and, if necessary, changes the preload level.
Thus, the inventive numerical control apparatus can correct the axial displacement of the main spindle caused due to the change in the preload level as well as the axial displacement of the main spindle occurring in accordance with the rotation speed, so that the machine tool has an improved machining accuracy.